1. Field of the Invention
The present invention relates to a method of manufacturing a III-V group compound semiconductor thin film by means of metal organics chemical vapor deposition (MOCVD), more particularly, to a method of growing a p-type GaAs layer or AlGaAs layer.
2. Description of the Related Art
The MOCVD method is widely employed for growing a III-V group compound semiconductor film. If the supply ratio of the V-group element material to the III-group element material, i.e., V/III ratio, is low in the manufacture of the compound semiconductor film, deterioration of the surface morphology and reduction in the film-growth rate are brought about simultaneously. To overcome the difficulty, it was customary in the past to set the V/III ratio at a level high enough to prevent the deterioration of the surface morphology. Consequently, the efficiency of utilizing the V-group material was very low in the past. It is known to the art to thermally decompose the V-group material in order to improve the efficiency of utilizing the V-group material. However, this method is incapable of achieving an epitaxial growth of the III-V group compound semiconductor of a high quality and, thus, is not practical.
It was conventional to use Be, Zn, Mg, or Cd as a p-type dopant of the III-V group compound semiconductor. Beryllium certainly permits doping at a high concentration, but is highly poisonous. On the other hand, each of Zn, Mg and Cd exhibits a high vapor pressure, is low in doping efficiency under high growth temperatures, is likely to be adsorbed on the wall of a pipe, and tends to exhibit a memory effect that intake of the raw material gas is delayed, resulting in failure to obtain abrubt dopant profiles.
Carbon, which is used as an acceptor in a III-V group compound semiconductor, is known to exhibit good electrical properties. However, only a few reports have been published to data concerning a dopant material suitable for achieving a carbon doping in the MOCVD method. Under the circumstances, it was very difficult to manufacture a p-type GaAs or AlGaAs layer having a high carrier concentration by the MOCVD method.
As a method of achieving carbon doping without using new dopant material, it is proposed to alternately supply a trimethylgallium (TMG) gas and an arsine (AsH.sub.3) gas in the step of growing a GaAs layer by the MOCVD method. In this method, however, a heavy burden is given to, for example, a valve mounted in the gas supply system because it is necessary to switch frequently the supply of the two different gases. Further, it is necessary to sufficiently evacuate the reaction furnace in switching the gas supply. In other words, a long gas switching time is required, leading to a low growth rate of a thin film.
To reiterate, carbon is an effective p-type impurity used in a GaAs or AlGaAs semiconductor. However, a suitable dopant material for achieving carbon doping in the MOCVD method has not yet been found. On the other hand, the method of growing a p-type GaAs layer while alternately supplying TMC and AsH.sub.3 gases necessitates a frequent switching of the gas supply, leading to a heavy burden to the apparatus and to a low growth rate of the GaAs layer. Under the circumstances, it was very difficult to manufacture a p-type GaAs or AlGaAs layer having a high carrier concentration by the MOCVD method.